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  对象的内存布局
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  <h4 id="一、概述"><a href="#一、概述" class="headerlink" title="一、概述"></a>一、概述</h4><ol>
<li><p>不同的对象有着不同的对象大小，理解java对象的内存布局知识，有利于对对象内存有更清晰的理解与控制。</p>
</li>
<li><p>众所周知synchronized与对象头有着莫大的渊源，了解了java对象头，有利于理解synchronized原理。</p>
</li>
</ol>
<h4 id="二、环境准备"><a href="#二、环境准备" class="headerlink" title="二、环境准备"></a>二、环境准备</h4><p>os:  win_x64</p>
<p>java环境:</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">java version &quot;1.8.0_201&quot;</span><br><span class="line">Java(TM) SE Runtime Environment (build 1.8.0_201-b09)</span><br><span class="line">Java HotSpot(TM) 64-Bit Server VM (build 25.201-b09, mixed mode)</span><br></pre></td></tr></table></figure>



<p>maven引入用于对象内存分析的工具</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">    &lt;dependency&gt;</span><br><span class="line">        &lt;groupId&gt;org.openjdk.jol&lt;/groupId&gt;</span><br><span class="line">        &lt;artifactId&gt;jol-core&lt;/artifactId&gt;</span><br><span class="line">        &lt;version&gt;0.9&lt;/version&gt;</span><br><span class="line">    &lt;/dependency&gt;</span><br><span class="line">    </span><br><span class="line"><span class="comment">//示例</span></span><br><span class="line">Object o = <span class="keyword">new</span> Object();</span><br><span class="line">System.out.println(ClassLayout.parseInstance(o).toPrintable());</span><br></pre></td></tr></table></figure>



<h4 id="三、对象的内存布局"><a href="#三、对象的内存布局" class="headerlink" title="三、对象的内存布局"></a>三、对象的内存布局</h4><h6 id="对象大小"><a href="#对象大小" class="headerlink" title=" 对象大小"></a><img src="https://pic.downk.cc/item/5f7ee14e1cd1bbb86bb8bd91.png" alt="img"> 对象大小</h6><p>​    1. object对象大小</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Object o = <span class="keyword">new</span> Object();</span><br><span class="line">System.out.println(ClassLayout.parseInstance(o).toPrintable());</span><br></pre></td></tr></table></figure>

<p><img src="https://pic.downk.cc/item/5f7ee1d11cd1bbb86bb8df24.png" alt="IMG"></p>
<p>​    这里在64位机器上默认会开启指针压缩的功能 -XX:+UseCompressedOops ，这里我关了试试</p>
<p>​    增加jvm运行参数 -XX:-UseCompressedOops <img src="https://pic.downk.cc/item/5f7ee2631cd1bbb86bb91525.png" alt="IMG"></p>
<p>  所以实际上64位机器上关闭指针压缩的话，class pointer占用8字节。</p>
<p>​    2. 普通对象</p>
   <figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Student</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> String name = <span class="string">&quot;jerry&quot;</span>;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> age = <span class="number">17</span>;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> List&lt;String&gt; course;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> String address = <span class="string">&quot;chengDu&quot;</span>;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">final</span> String like = <span class="string">&quot;game&quot;</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        Student o = <span class="keyword">new</span> Student();</span><br><span class="line">        System.out.println(ClassLayout.parseInstance(o).toPrintable());</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure>

   <figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">com.hs.test.Student object internals:</span><br><span class="line"> OFFSET  SIZE               TYPE DESCRIPTION                               VALUE</span><br><span class="line">      0     4                    (object header)                           01 00 00 00 (00000001 00000000 00000000 00000000) (1)</span><br><span class="line">      4     4                    (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)</span><br><span class="line">      8     4                    (object header)                           44 c1 00 f8 (01000100 11000001 00000000 11111000) (-134168252)</span><br><span class="line">     12     4                int Student.age                               17</span><br><span class="line">     16     4   java.lang.String Student.name                              (object)</span><br><span class="line">     20     4   java.lang.String Student.like                              (object)</span><br><span class="line">Instance size: 24 bytes</span><br><span class="line">Space losses: 0 bytes internal + 0 bytes external &#x3D; 0 bytes total</span><br></pre></td></tr></table></figure>

<p>   这里成员变量大小都会在对象内存布局中展示出来,没有静态变量。</p>
<p>​    3.  数组对象</p>
   <figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span>[] o = &#123;<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>,<span class="number">3</span>,<span class="number">4</span>,<span class="number">5</span>,<span class="number">6</span>&#125;;</span><br><span class="line">System.out.println(ClassLayout.parseInstance(o).toPrintable());</span><br></pre></td></tr></table></figure>

   <figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">[I object internals:</span><br><span class="line"> OFFSET  SIZE   TYPE DESCRIPTION                               VALUE</span><br><span class="line">      <span class="number">0</span>     <span class="number">4</span>        (object header)                           <span class="number">01</span> <span class="number">00</span> <span class="number">00</span> <span class="number">00</span> (<span class="number">00000001</span> <span class="number">00000000</span> <span class="number">00000000</span> <span class="number">00000000</span>) (<span class="number">1</span>)</span><br><span class="line">      <span class="number">4</span>     <span class="number">4</span>        (object header)                           <span class="number">00</span> <span class="number">00</span> <span class="number">00</span> <span class="number">00</span> (<span class="number">00000000</span> <span class="number">00000000</span> <span class="number">00000000</span> <span class="number">00000000</span>) (<span class="number">0</span>)</span><br><span class="line">      <span class="number">8</span>     <span class="number">4</span>        (object header)                           <span class="number">6d</span> <span class="number">01</span> <span class="number">00</span> f8 (<span class="number">01101101</span> <span class="number">00000001</span> <span class="number">00000000</span> <span class="number">11111000</span>) (-<span class="number">134217363</span>)</span><br><span class="line">     <span class="number">12</span>     <span class="number">4</span>        (object header)                           <span class="number">07</span> <span class="number">00</span> <span class="number">00</span> <span class="number">00</span> (<span class="number">00000111</span> <span class="number">00000000</span> <span class="number">00000000</span> <span class="number">00000000</span>) (<span class="number">7</span>)</span><br><span class="line">     <span class="number">16</span>    <span class="number">28</span>    <span class="keyword">int</span> [I.&lt;elements&gt;                             N/A</span><br><span class="line">     <span class="number">44</span>     <span class="number">4</span>        (loss due to tde next object alignment)</span><br><span class="line">Instance size: <span class="number">48</span> bytes</span><br><span class="line">Space losses: <span class="number">0</span> bytes internal + <span class="number">4</span> bytes external = <span class="number">4</span> bytes total</span><br></pre></td></tr></table></figure>

<p>   这里的</p>
   <figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">12     4        (object header)                           07 00 00 00 (00000111 00000000 00000000 00000000) (7)</span><br></pre></td></tr></table></figure>

<p>   显示了对象头包含了数组的length;</p>
<blockquote>
<p>总结：对于对象的内存大小（64位默认开启指针压缩情况下）：</p>
<ul>
<li>普通对象：<ul>
<li>markWord 8byte</li>
<li>klass pointer 4byte</li>
<li>成员变量实际情况来占用</li>
<li>padding = 8的倍数的余数</li>
</ul>
</li>
<li>数组对象：<ul>
<li>多了个length占用4个字节，其它情况与普通对象一致。</li>
</ul>
</li>
</ul>
</blockquote>
<h4 id="四、对象头"><a href="#四、对象头" class="headerlink" title="四、对象头"></a>四、对象头</h4><p>参考<a target="_blank" rel="noopener" href="http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee27509/src/share/vm/oops/markOop.hpp">markOop.hpp</a></p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//  32 bits:</span></span><br><span class="line"><span class="comment">//  --------</span></span><br><span class="line"><span class="comment">//             hash:25 ------------&gt;| age:4    biased_lock:1 lock:2 (normal object)</span></span><br><span class="line"><span class="comment">//             Javatdread*:23 epoch:2 age:4    biased_lock:1 lock:2 (biased object)</span></span><br><span class="line"><span class="comment">//             size:32 ------------------------------------------&gt;| (CMS free block)</span></span><br><span class="line"><span class="comment">//             PromotedObject*:29 ----------&gt;| promo_bits:3 -----&gt;| (CMS promoted object)</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">//  64 bits:</span></span><br><span class="line"><span class="comment">//  --------</span></span><br><span class="line"><span class="comment">//  unused:25 hash:31 --&gt;| unused:1   age:4    biased_lock:1 lock:2 (normal object)</span></span><br><span class="line"><span class="comment">//  Javatdread*:54 epoch:2 unused:1   age:4    biased_lock:1 lock:2 (biased object)</span></span><br><span class="line"><span class="comment">//  PromotedObject*:61 ---------------------&gt;| promo_bits:3 -----&gt;| (CMS promoted object)</span></span><br><span class="line"><span class="comment">//  size:64 -----------------------------------------------------&gt;| (CMS free block)</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">//  unused:25 hash:31 --&gt;| cms_free:1 age:4    biased_lock:1 lock:2 (COOPs &amp;&amp; normal object)</span></span><br><span class="line"><span class="comment">//  Javatdread*:54 epoch:2 cms_free:1 age:4    biased_lock:1 lock:2 (COOPs &amp;&amp; biased object)</span></span><br><span class="line"><span class="comment">//  narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 -----&gt;| (COOPs &amp;&amp; CMS promoted object)</span></span><br><span class="line"><span class="comment">//  unused:21 size:35 --&gt;| cms_free:1 unused:7 ------------------&gt;| (COOPs &amp;&amp; CMS free block)</span></span><br></pre></td></tr></table></figure>

<p>整理了下：如下64位MarkWord的存储内容，32位存储内容及状态都与64位一致，只是占用大小区别。</p>
<table>
    <tr bgcolor="#66ce78">
        <td>锁状态</td>
        <td>25bit</td>
        <td>31bit</td>
        <td>1bit</td>
        <td>4bit</td>
        <td>1bit偏向锁位</td>
        <td colspan="2">2bit锁标志位</td>
    </tr>
    <tr >
        <td>无状态(new)</td>
        <td>unused</td>
        <td>hashCode(如果有调用)</td>
        <td>unused</td>
        <td>分代年龄</td>
        <td>0</td>
        <td>0</td>
        <td>1</td>
    </tr>
    <tr bgcolor="#49a7d6">
        <td>锁状态</td>
        <td>54bit</td>
        <td>2bit</td>
        <td>1bit</td>
        <td>4bit</td>
        <td>1bit偏向锁位</td>
        <td colspan="2">2bit锁标志位</td>
    </tr>
    <tr >
        <td>偏向锁</td>
        <td>当前线程指针JavaThread*</td>
        <td>Epoch</td>
        <td>unused</td>
        <td>分代年龄</td>
        <td>1</td>
        <td>0</td>
        <td>1</td>
    </tr>
        <tr bgcolor="#597dd2">
        <td>锁状态</td>
        <td colspan="5">62bit</td>
        <td colspan="2">2bit锁标志位</td>
    </tr>
    <tr >
        <td>轻量级锁/自旋锁/无锁</td>
        <td colspan="5">指向线程栈中Lock Record的指针</td>
        <td>0</td>
        <td>0</td>
    </tr>
    <tr>
        <td>重量级锁</td>
        <td colspan="5">指向互斥量(重量级锁)的指针</td>
        <td>1</td>
        <td>0</td>
    </tr>
    <tr>
        <td>GC标记信息</td>
        <td colspan="5">CMS标记过程用到的标记信息</td>
        <td>1</td>
        <td>1</td>
    </tr>
</table>



<h6 id="无锁状态下的markWord"><a href="#无锁状态下的markWord" class="headerlink" title="无锁状态下的markWord"></a>无锁状态下的markWord</h6><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">Object o = <span class="keyword">new</span> Object();</span><br><span class="line">System.out.println(<span class="string">&quot;[hasCode before]&quot;</span>);</span><br><span class="line">System.out.println(ClassLayout.parseInstance(o).toPrintable());</span><br><span class="line">System.out.println(<span class="string">&quot;[hasCode after]&quot;</span>);</span><br><span class="line">o.hashCode();</span><br><span class="line">System.out.println(ClassLayout.parseInstance(o).toPrintable());</span><br></pre></td></tr></table></figure>

<p><img src="https://pic.downk.cc/item/5f81da4d1cd1bbb86b7c9501.jpg" alt="IMG"></p>
<p>这里是小端存储，倒序查看二进制位</p>
<ul>
<li>这里在hashCode后，31bit为hashCode占用，剩余的为:</li>
</ul>
<p><img src="https://pic.downk.cc/item/5f8317151cd1bbb86b658262.jpg" alt="IMG"></p>
<p>这里GC年龄只有4位，所以说GC年龄最大15岁。</p>
 
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